GNU Emacs can be programmed to emulate (more or less) most other
editors. Standard facilities can emulate these:

EDT (DEC VMS editor)

Turn on EDT emulation with M-x edt-emulation-on. M-x
edt-emulation-off restores normal Emacs command bindings.
Most of the EDT emulation commands are keypad keys, and most standard
Emacs key bindings are still available. The EDT emulation rebindings
are done in the global keymap, so there is no problem switching
buffers or major modes while in EDT emulation.

vi (Berkeley editor)

Viper is the newest emulator for vi. It implements several levels of
emulation; level 1 is closest to vi itself, while level 5 departs
somewhat from strict emulation to take advantage of the capabilities of
Emacs. To invoke Viper, type M-x viper-mode; it will guide you
the rest of the way and ask for the emulation level.

vi (another emulator)

M-x vi-mode enters a major mode that replaces the previously
established major mode. All of the vi commands that, in real vi, enter
"input" mode are programmed instead to return to the previous major
mode. Thus, ordinary Emacs serves as vi's "input" mode.
Because vi emulation works through major modes, it does not work
to switch buffers during emulation. Return to normal Emacs first.
If you plan to use vi emulation much, you probably want to bind a key
to the vi-mode command.

vi (alternate emulator)

M-x vip-mode invokes another vi emulator, said to resemble real vi
more thoroughly than M-x vi-mode. "Input" mode in this emulator
is changed from ordinary Emacs so you can use ESC to go back to
emulated vi command mode. To get from emulated vi command mode back to
ordinary Emacs, type C-z.
This emulation does not work through major modes, and it is possible
to switch buffers in various ways within the emulator. It is not
so necessary to assign a key to the command vip-mode as
it is with vi-mode because terminating insert mode does
not use it.
For full information, see the long comment at the beginning of the
source file, which is `lisp/vip.el' in the Emacs distribution.